Exploding Custard Powder

Not only is custard a chemical cornucopia and a great case study for fluid dynamics, custard powder sometimes demonstrates another physical phenomenon, and that’s combustion. Custard powder is flammable, and Chris Smith is out in the car park with Dave Ansell to prove it to you...

Dave - Well custard is a fuel like any other, like wood or oil. It has got hydrogen and carbon in it. You burn it, those will react with oxygen and give off energy. Normally, this will happen really slowly, in the same way that if you have a big lump of wood it will take ages to burn, twigs will take minutes, if you chop it down to a piece of paper it might burn in seconds. But the real thing about cornflour, which is in custard powder, is that the particles are microns across, maybe 10 microns across. So if you can get the oxygen to them, they'll burn very, very quickly.

Chris - And to put that into perspective, microns being tiny, those are the size of particles that come out of the exhaust pipes of cars. These are really tiny particles.

Dave - Very, very tiny. That's a millionth of a metre - is a micron.

Chris -  And does the same physics apply to pretty much anything that's a fuel source, because you mentioned carbon, hydrogen and oxygen in the molecules for example. So basically anything that can work as a fuel, the physics applies?

Dave - Yeah. So if you can break up any fuel into very, very small particles and mix it with air it will burn very quickly. That's not just things with carbon and hydrogen in them; even things like titanium will do the same thing. So there was a titanium factory which blew up in 2014/2015 in the States, where they got some titanium dust, some spark ignited it, and the whole thing went off, bang. 

Chris - And essentially, in terms of a real world application, a diesel engine or an engine that's making a spray of petrol vapour inside the cylinder, air-fuel mixtures work in the same way, isn't it?

Dave - Yeah, basically you're splitting up your fuel into very, very small particles, you mix it well, it'll burn. And the smaller the particles, the quicker it will burn. Probably the extreme examples of this is gas. If you mix gas with air really well, it will go off with a really, really violent bang.

Chris - And now you're going to show us how it works. You've lit a gas torch in front of us, which is keeping us warm on this very cold evening. You have a thing that looks like a gentleman's pipe from the 1800s, but with an exceptionally long piece of plastic tube on the end of it, going into it. And you have loaded the bowl of the pipe with...

Dave - This is custard powder.

Chris - Okay. And what are you going to do?

Dave - So basically we want to suspend this in the air, so I'm going to blow air into this pipe, the pipe's set off-centre so it kind of spirals inside the bowl of the pipe, so it mixes with air very very efficiently. And...

Chris - Shall we count down? Three, two, one, Dave's going to blow it in the flame...

*FIREBALL*

Chris - I don't have many hair follicles left now, or eyebrows, but there was an enormous fireball - basically that was about three feet across - and it went in a fraction of a second.

Dave - Yeah. It burns very, very quickly, lots of heat released. If that was a whole building going up then you'd have serious problems.

Chris - And it's the heat that's released from that, that then causes the air to expand and that of course is the explosive force?

Dave - Yeah. As with any explosion, heat causing gas to expand, that then hits things, blows apart buildings. I mean, whole buildings have been completely destroyed by this effect.

Chris - Dave, thank you very much for demonstrating it for us. Demonstrator extraordinaire Dave Ansell.